Recently, there have been demands for the display of word processors, notebook computers, etc. to have a lighter weight and a thinner thickness and to consume less electric power, and a superior liquid crystal display device is expected to be developed for a flat display satisfying these requirements. A liquid crystal display device is provided with a liquid crystal aligning film having a predetermined pretilt angle so as to align liquid crystal molecules to a given direction. As a method for preparing such aligning films, a rubbing process wherein a thin film of a polymeric compound such as a polyimide formed on a substrate is rubbed toward a certain direction with a cloth such as rayon, a process wherein silicon dioxide is subjected to oblique evaporation, etc. are known.
However, although the rubbing process has been widely applied in the industrial field as a convenient and inexpensive process, it has the problems of the formation of dusts, the generation of static electricity and the like.
The oblique evaporation process costs too much to be scaled up.
Under these circumstances, a photo-orientation process has recently attracted attention of the industry. In such a process for the preparation of an aligning film according to this photo-orientation method, a thin film of a photosensitive polymeric compound is formed on a substrate and a polarized ultraviolet light or a laser light is irradiated thereto, whereby only the photosensitive groups in line with the irradiated polarized light may photochemically react to develop anisotropy on the aligning film and align liquid crystal molecules. This process can provide the advantages of no generation of static electricity or no contamination of impurities because the other members are not in contact with the thin film coated over the substrate.
Several photo-aligning films utilizing photodimerization of a polarized light irradiated to polyvinyl cinnamate and derivatives thereof are disclosed, for example, in M. Schadt et al., Jpn. J. Appl. Phys., 31, 2155 (1992) or Japanese Patent No. 2608661. However, these aligning films have the drawbacks of a low heat stability, an inferior retention of shape and an easily turbulent alignment of liquid crystal.
Vinyl polymers having arylmaleimide residues in the side chain thereof are disclosed as a photosensitive group having a higher sensitivity in Japanese Patent Publication No. 13198/1976 and so on, but no examples of utilizing them as an aligning film for liquid crystal display device is disclosed therein.
The present inventors have made earnest studies, and as a result, have found out that a polyimide having an .beta.,.beta.-substituted maleimide group in the side chain thereof may be rapidly dimerized in the side chain thereof at a high sensitivity, and that the crosslinked film is excellent in a heat stability and a retention of shape and also shows a favorable alignment of liquid crystal, upon which this invention has been completed.